INVENTORIES (Narrative) (Details) - USD ($) $ in Millions  | 
12 Months Ended | |
|---|---|---|
Dec. 31, 2017  | 
Dec. 31, 2016  | 
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| Inventory [Line Items] | ||
| Cost of Goods Sold | $ 6.2 | $ 8.8 | 
| LCM inventory adjustment | 1.8 | 0.0 | 
| U.S. Iron Ore [Member] | ||
| Inventory [Line Items] | ||
| Inventory, LIFO Reserve | $ 96.2 | $ 78.5 | 
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- Definition Effect of inventory reduction No definition available. 
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- Definition LCM inventory adjustment No definition available. 
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- Definition Amount by which inventory stated at last-in first-out (LIFO) is less than (in excess of) inventory stated at other inventory cost methods. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Details 
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